Double counter-shafts are used, on the one hand, in gearboxes in which it is desired, with a given length of gearbox, to obtain more gear ratios than what is practically possible in a conventional gearbox design with one counter-shaft, and, on the other hand, in so-called powershift gearboxes, i.e. gearboxes with double clutches, each cooperating with an individual counter-shaft. In such gearboxes the gear speeds are preselected and shifting is thereafter effected by disengaging one clutch and engaging the other clutch.
Various solutions are known for achieving a compact gearbox design of the powershift type with a large number of gear ratios. In one type which is known by SE 8700583-1 (corresponding to US-A-4,876,907) two concentric input shafts are used driving individual counter-shafts which can be engaged alternately with the aid of a pair of multiple disc clutches arranged in series outside the gearbox. Synchronization is achieved with the aid of a central synchronizing clutch on one of the counter-shafts. In another gearbox type with double counter-shafts, which is known by e.g. GB-A-2,110,324, a single input shaft drives simultaneously two gear wheels which, via individual clutches arranged inside the housing on the respective counter-shafts, can be locked to its shaft and establish a drive train between the input shaft and the output shaft via either counter-shaft. The synchronization is achieved with the aid of a two-part synchronizing device, which in this version is formed by a synchronizing clutch on either counter-shaft. In this gearbox, the torque is transmitted from the input shaft to the output shaft in all gear ratios Via a counter-shaft. The gearbox thus has no direct drive gear ratio.